U.S. patent application number 10/571235 was filed with the patent office on 2009-01-01 for high temporal resolution optical sampler and sampling method.
This patent application is currently assigned to MARCONI COMMUNICATIONS SPA. Invention is credited to Antonella Bogoni, Filippo Ponzini, Luca Poti.
Application Number | 20090001963 10/571235 |
Document ID | / |
Family ID | 34260017 |
Filed Date | 2009-01-01 |
United States Patent
Application |
20090001963 |
Kind Code |
A1 |
Bogoni; Antonella ; et
al. |
January 1, 2009 |
High Temporal Resolution Optical Sampler and Sampling Method
Abstract
An optical sampler with high temporal resolution comprises a
TOAD device (11) with a loop optical path (12) at the inlet of
which is input an optical signal to be sampled Si of duration
T.sub.p and along which path is arranged a point (13) of input of
an optical control signal Sc produced by a source (16) and
appropriately delayed by a delay line (23) to change on command the
temporal position of the TOAD transmittance window compared to the
signal to be sampled. In the loop there is also a nonlinear device
(14) The sampler also comprises control means (17) to command the
delay line (23) to move step by step said transmittance window and
make it run on the signal to be sampled. Measurement means (18)
measure the mean power transmitted at the TOAD output for each
window position and processing means (19) perform the derivative on
the mean powers found for each window position thus finding samples
representing the input signal S.sub.i.
Inventors: |
Bogoni; Antonella; (Montova,
IT) ; Poti; Luca; (Pisa, IT) ; Ponzini;
Filippo; (Bedonia, IT) |
Correspondence
Address: |
KIRSCHSTEIN, OTTINGER, ISRAEL;& SCHIFFMILLER, P.C.
425 FIFTH AVENUE, 5TH FLOOR
NEW YORK
NY
10016-2223
US
|
Assignee: |
MARCONI COMMUNICATIONS SPA
Genova
IT
|
Family ID: |
34260017 |
Appl. No.: |
10/571235 |
Filed: |
September 6, 2004 |
PCT Filed: |
September 6, 2004 |
PCT NO: |
PCT/EP2004/052058 |
371 Date: |
September 8, 2008 |
Current U.S.
Class: |
324/121R |
Current CPC
Class: |
G02F 1/3519 20130101;
G01J 11/00 20130101 |
Class at
Publication: |
324/121.R |
International
Class: |
G01R 13/24 20060101
G01R013/24 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 9, 2003 |
IT |
MI2003A001726 |
Claims
1-11. (canceled)
12. An optical sampler with high temporal resolution, comprising: a
terabit optical asymmetric demultiplexer (TOAD) device including a
loop optical path having an input for inputting an optical signal
to be sampled, the optical signal having a duration, a source for
generating an optical control signal and for inputting the control
signal to the optical path, a delay line for delaying the control
signal to change a temporal position of a transmittance window, and
control means in the optical path; measurement means for measuring
a mean power at an output of the optical path for each window
position; delay control means for controlling the delay line to
move step-by-step the transmittance window having a window duration
greater than the duration of the optical signal in a temporal
interval at least equal to the duration of the optical signal; and
processing means for performing a derivative on the temporal
interval of the mean powers measured to thus obtain data samples
representing the optical
13. The sampler in accordance with claim 12, in that the processing
means is operative for performing a discrete derivative in time on
a sequence of data samples representing the mean powers for each
window position, the derivative being defined as: where d.sub.k is
the derived sequence, a.sub.k (with k from 0 to N) is the data
sample sequence incoming to the processing means, N+1 is the number
of data samples and .DELTA.t is the temporal delay between two
adjacent samples.
14. The sampler in accordance with claim 12, in that the processing
means includes a low-pass filter for filtering the measured mean
powers before the derivative is performed.
15. The sampler in accordance with claim 14, in that the low-pass
filter performs a numerical filtering on a sequence of the data
samples representing the mean powers for each window position.
16. The sampler in accordance with claim 15, in that the low-pass
filter is a Butterworth low-pass filter of a first order with a
cutoff frequency from 2% to 10% of a frequency of the sequence of
the data samples.
17. The sampler in accordance with claim 16, in that the cutoff
frequency is around 5% of the frequency of the sequence of the data
samples.
18. The sampler in accordance with claim 12, and additional means
for receiving samples representative of the optical signal, and for
processing and displaying a graph representing magnitudes of the
optical signal.
19. The sampler in accordance with claim 12, in that the control
means is an SOA amplifier.
20. The sampler in accordance with claim 12, in that a variable
delay line for modifying the transmittance window is provided in
the optical path upstream of the control means.
21. A high temporal resolution sampling method for an optical
signal, comprising the steps of: a) applying the optical signal to
a terabit optical asymmetric demultiplexer (TOAD) device having a
transmittance window of a window duration greater than a duration
of the optical signal to be sampled; b) measuring a mean power
transmitted at an output of the TOAD device for each window
position; c) moving the transmittance window step-by-step in a
temporal interval equal at least to the duration of the optical
signal to be sampled; and d) performing a derivative on the
temporal interval of the mean powers measured to obtain data
samples representative of the optical signal.
22. The method in accordance with claim 21, and the additional step
of performing a low-pass filtering on the measured data samples
before performing the derivative.
Description
[0001] The present invention relates to a sampling method and a
low-cost, simple and stable optical sampler with high temporal
resolution. In particular, the sampler can easily reach a
resolution of a few hundred femtoseconds with a very simple scheme.
The sampler is thus suited to evaluation of the shape of pulses
with duration on the order of picoseconds and even less.
[0002] In recent years, interest in the use of ultra-short optical
pulses has increased because of the requirement for high bit-rate
transmission systems and characterization of the dynamics of many
ultra-fast nonlinear optical devices has increased.
[0003] To seek to satisfy the need for analysis of optical signals
beneath the temporal resolution limit of modern oscilloscopes,
optical samplers have been proposed. Different optical sampling
techniques have been proposed but they are generally costly and
very complex or they have stability and temporal resolution
limitations. In particular, it is very difficult to reach temporal
resolutions on the order of magnitude of the picosecond or
less.
[0004] The general purpose of the present invention is to remedy
the above-mentioned shortcomings by making available a sampling
method and a low cost, simple and stable optical sampler with high
temporal resolution on the order of a few hundred femtoseconds.
[0005] In view of this purpose it was sought to provide in
accordance with the present invention a high temporal resolution
optical sampler comprising a TOAD device with a loop optical path
at the input of which is fed in an optical signal to be sampled
S.sub.i of duration T.sub.p and along which path is arranged a
point of input of an optical control signal S.sub.c produced by a
source and appropriately delayed compared to the signal to be
sampled by means of a delay line (23) to change on command the
temporal position of the TOAD transmittance window compared to the
signal to be sampled with there also being in the loop a nonlinear
device (14) and there are also control means to command the delay
line to move step by step said transmittance window of a duration T
greater than T.sub.p in a temporal interval at least equal to the
duration T.sub.p of the signal to be sampled to run on it and means
of measurement for measuring the mean power transmitted at the TOAD
output for each position of the window and means of processing
which execute the derivative on said temporal interval of the mean
powers measured and thus obtaining samples representative of the
signal S.sub.i input.
[0006] Again in accordance with the present invention it was sought
to realize a sampling method with high temporal resolution sampling
method of an optical signal S.sub.i comprising the steps of
applying the signal S.sub.i to a TOAD device having a transmittance
window of duration T greater than the signal S.sub.i to be sampled
and moving the transmittance window step by step in a temporal
interval at least equal to the duration T.sub.p of the signal to be
sampled to run thereon and measure the mean power transmitted at
the output of the TOAD device for each window position and execute
the derivative on said temporal interval of the mean measured
powers to obtain samples representative of the entered signal.
[0007] To clarify the explanation of the innovative principles of
the present invention and its advantages compared with the prior
art there is described below with the aid of the annexed drawings a
possible embodiment thereof by way of non-limiting example applying
said principles. In the drawings:
[0008] FIG. 1 shows a block diagram of an optical sampler realized
in accordance with the present invention, and
[0009] FIG. 2 shows a graph of the operating principal of the
sampler of FIG. 1.
[0010] With reference to the figures, FIG. 1 shows designated
generally by reference number 10 an optical sampler in accordance
with the present invention. The optical sampler 10 is based on a
Terabit Optical Asymmetric Demultiplexer (TOAD) 11. For this reason
the sampler will be called here TOS (TOAD-based Optical
Sampler).
[0011] The TOAD is a known device based on the Sagnac
interferometric principle and is used in the prior art to
demultiplex composite signals, i.e. extract a desired optical
channel from a plurality of optical channels mutually temporally
multiplexed. This function is however totally unused in the sampler
in accordance with the present invention.
[0012] The TOAD, as shown in FIG. 1, consists of a loop reflection
path 12 with a coupler 13 (type 2.times.2) and a nonlinear member
14 both arranged along the loop. Advantageously the nonlinear
member 14 is thrown out of phase by a distance .DELTA.x from the
median point of the loop. A check pulse S.sub.c is produced by an
appropriate known source 16 and is injected into the loop 12.
[0013] In the prior art, TOAD devices were proposed for fast
demultiplexing operations in Optical Time Division Multiplexing
(OTDM) systems because, through a control pulse inserted in the
loop through the coupler, they permit opening a transmittance
window whose temporal duration depends only on the distance
.DELTA.x. Indeed, the control pulse saturates the nonlinear member
so that if two counterpropagating signal components pass through
the device respectively before and after the control pulse, they
undergo different phase modulation. On the contrary, they are
affected by a same phase modulation.
[0014] These two different conditions permit obtaining two
different transmission values towards the outlet of the
interferometric structure, i.e. at the end of the loop opposite
that of input of the signal to be demultiplexed. The structure and
operating principles of a TOAD will not be shown here or described
in detail as they are well known to those skilled in the art.
[0015] By decreasing the distance .DELTA.x the relative delay
between the intersection times of the two counterpropagating
signals can be reduced and consequently the temporal length of the
transmission window is shortened.
[0016] Distance .DELTA.x can be changed by means of a known
variable optical delay line 15 inserted in the loop.
[0017] The rise and fall times of the beginning and ending fronts
of the transmittance window depend on the nonlinear member 14. For
the use in accordance with the present invention it was found
advantageous to use a known Semiconductor Optical Amplifier (SOA)
as the nonlinear member. In this case, indeed, the beginning and
ending fronts of the window depend on the saturation time of the
SOA and the duration of the control pulse. Appropriately optimizing
the control signal power level to reduce the TOAD transmittance
window transition time, the basic limit becomes only the SOA
saturation time which is a few hundred femtoseconds.
[0018] As known, a sampling process requires an ultra-short
temporal window compared to the amplitude of the pulse to be
reproduced. As just said, the rising and falling fronts do not
constitute a problem as they can be obtained sufficiently steep.
The minimum duration of the window is however limited by the
nonlinear member propagation time which is greater than or at least
comparable to the duration of the signals it is desired to sample.
This is sufficient for conventional use of the demultiplexers for
which the TOAD is ordinarily used. But it is not at all acceptable
for use as a sampler.
[0019] In accordance with the principles of the present invention
the TOAD was found useable as a sampler even though considering an
amplitude of the transmittance window broader than the entire pulse
to be sampled on condition that it be sufficiently narrow to reject
adjacent pulses. Appropriate pulse duration adjustment can be done
manually by means of the delay line 15.
[0020] Indeed, by moving the window above an appropriate interval
in time, it was found possible to realize an integration of the
signal pulse. Indeed, as may be seen in FIG. 2, by moving the
window step by step by an amount .tau. in a time interval equal to
the entire duration T.sub.p of the pulse to be sampled, from the
central instant t.sub.0-T/2 and measuring the mean power
transmitted at the outlet for each position of the window the
following function is found:
X(.tau.)=k.intg.x(t)dt
where x(t) is the individual pulse shape function and k is a
constant.
[0021] Correct movement of the window depending on the signal to be
measured is found by controlling appropriately by means of a known
control device 17 a delay line 23 arranged to delay the control
signal compared to the signal to be sampled. The functions of the
known control device 17 and the processing unit 19 are carried out
by a PC (not shown). Using LABVIEW.TM. software the PC imparts a
desired delay to the delay line 23. The PC then measures the delay
which has been implemented by the delay line 23 to verify that it
is correct before indicating to the processing unit 19 when to
acquire the data. This process is iterated until the delay which
has been implemented is substantially equal to the desired delay.
Once the desired delay has been attained the control device 17 then
authorises the processing unit 19 to proceed and to perform the
data filtering at 21 and to obtain the derivative at 22.
[0022] The mean power X(.tau.) of the signal is found by an
appropriate known means or power measurer 18 arranged at the outlet
of the TOAD and which advantageously supplies sampled results in
numerical form for subsequent processing.
[0023] To find samples representative of the shape of the
individual pulse being tested it is sufficient to derive the curve
X(.tau.) at the outlet of the measurer. Resolution is limited only
by transition time of the transmittance window and by the
resolution of the optical delay line which moves the control pulses
compared to the signal in order to find the temporal running of the
window.
[0024] The signal X(.tau.) is therefore sent to a processing unit
19 which performs the derivative of the incoming signal to then
send its result to a display unit 20 for example.
[0025] A time discrete derivative is simple to find and is defined
as:
N 1 .A-inverted. k d k = a k - a k - 1 .DELTA. t ##EQU00001##
where d.sub.k is the derivative sequence, a.sub.k (with k from 0 to
N) the filtered sequence, N+1 the number of data samples and
.DELTA.t the temporal delay between two adjacent samples.
[0026] In comparison with fiber-based interferometric structures,
TOAD has many advantages such as a higher temporal stability and
insensitivity to polarization if an SOA independent of polarization
is used. Moreover, transition and transmittance times can be
significantly lower than control pulse amplitude.
[0027] To sum up, the physical limit of the sampling process
realized in accordance with the present invention depends only on
transmittance window rising time and consequently resolution is
fixed by SOA saturation time and therefore pulses with duration of
less than a picosecond and resolution of a few hundred femtoseconds
can be sampled. This is achieved without the need for ultra-short
optical sampling pulses.
[0028] As may be seen again in FIG. 1, to increase the quality of
the result it was found advantageous for the processing unit 19 to
include a low-pass filter 21 before the derivative calculation
means 22.
[0029] The filtering operation eliminates noise arising from
instability of the laser source and polarization residue dependent
upon the particular SOA used.
[0030] The data acquired, which represent integration of the pulse
shape, can have not negligible fluctuations because of the
measurement inaccuracies, source instability, pulse jitters et
cetera. Moreover, any use of a SOA dependent on polarization
produces an increase in data fluctuations acquired because of
polarization variations. These fluctuations compromise the
derivation operations in time. However, the high-frequency nature
of the noise contribution permits eliminating these measurement
degradation factors effectively by low-pass filtering.
[0031] From the experimental tests it was shown that efficiency in
noise reduction is independent of the filter form and in practice
depends only on its amplitude. Naturally, the filter band must be
optimized to reduce the above-mentioned fluctuations without
influencing the information.
[0032] For efficient noise rejection it was found advantageous to
use a Butterworth low-pass filter of the first order having a
cutting frequency from 2% to 10% of the frequency of the data
sample sequence. In particular, a figure around 5% of the data
sample sequence frequency was found advantageous. After the
filtering operation the data can be derived to find the pulse
shape.
[0033] It is now clear that the predetermined purposes have been
achieved by making available a simple and reliable sampler
permitting accurate sampling of ultra-short pulses. Comparative
tests among the results obtainable with a sampler in accordance
with the present invention and known samplers of undoubtedly higher
cost and complexity made clear the excellent performance of the
sampler in accordance with the present invention.
[0034] Naturally the above description of an embodiment applying
the innovative principles of the present invention is given by way
of non-limiting example of said principles within the scope of the
exclusive right claimed here. For example, in addition to being
used to display the shape of the sampled signal, the sample data
can be used for any other desired purpose and also further
processed. Naturally, the visualization obtained can be of other
magnitudes representing the entering signal and not only of its
amplitude in time.
* * * * *